Filament current control by a superposed dithering voltage



NOV. 26, 1968 BARBER ET AL V 3,413,517

FILAMENT CURRENT CONTROL BY A SUPERPOSED DIT HERING VOLTAGE Filed Jan. 13, 1967 EIEW,

51 LOW VOLTAGE DC POWER SUPPLY ";I l DIFFERENTIAL 48 58 M). AMPLIFIER 5a 19{}-50 men VOLTAGE POW? SUPPLY I si ELECTRON BEAM T"' FILAMENT CURRENLI 3 lNV ENTORS I FRED KURZWE|L,JR. ROBERT RUSSELL BABER.

1. m L B)! j MM ATTORNEY UnitediStates Patent Ofice 3 ,4 1 3 ,5 17 Patented Nov. 26, 1 968 3,413,517 FILAMENT CURRENT CONTROL BY A SUPERPOSED DITHERING VOLTAGE Robert Russell Barber, Paul Chialin Lang, and Karl Heinz Loetller, San Jose, and Fred Kurzweil, Jr., Saratoga,

Calif., assignors to International Business Machines Corporation, a corporation of New York Filed Jan. 13, 1967, Ser. No. 609,224 8 Claims. (Cl. 3l5106) ABSTRACT OF THE DISCLOSURE A cathode filament current control system for an electron beam generating device wherein the filament current is dithered and the change in electron emission is detected and compared with a reference signal for setting the filament current at a value always to operate the filament at a point just below the knee of the filament current vs. beam emission curve.

Cross-references This invention relates to such beam devices as that described in US. patent application Ser. No. 575,731, by K. H. Loeffler et al., entitled, Electron Optical Unit, and filed on Aug. 29, 1966, now patent No. 3,345,529, issued Oct. 3, 1967.

Background of the invention This invention is suitable for use in such electron beam and other particle beam devices as those used in electron beam recorders.

In devices utilizing a filament which is heated by the passage of current therethrough for generating a particle beam such as a beam of electrons, the operating life depends primarily upon the useful life of the filament. To obtain the electron flow, the filament is heated for increasing the thermal agitation thereby efiecting an electron emission. The thermal agitation of the filament also results in atoms being emitted serving to erode away the filament material, Thus, the filament life is shortened by evaporation of the filament material.

It might be supposed that filament life could be extended merely by making the filament larger. It is obvious that this approach would extend the time it takes for the material to evaporate. However, the larger size increases the current necessary for heating the filament. The increase in currentnecessary to heat the filament tothe emission temperatute is proportional to the square of the increase in diameter since the heating is proportional to the current density in the cross-sectional area of the material, thereby making the power requirements for the larger filament impractical. Also, frequently, a small point source for the beam is desired for formation of a high density beam, which smaller size is more difiicult to obtain from a physically large filament. The smaller point source is practically a necessity in data recording where a small beam is desired for scanning discrete areas of a memory element thereby increasing the amount of data that can be recorded in a given area by increasing the density at which the data can be recorded.

Further complicating the problem of controlling filament life is the fact that the operating parameters of the filament change during usage, due primarily to the fact that the physical size of the filament diminishes. Thus, as the filament size diminishes while the current supplied thereto is maintained constant, the evaporation process frequently is increased due to the greater heating effects of the higher current densities within the filament. On the other hand, if a constant voltage source is used to supply filament current, the resistance of the filament increases as the evaporation process decreases the material crosssection and a point can be reached before filament failure where the electron emission is not sufiicient for proper operation of the beam device. The problem of extending the filament life particularly is diflicult in devices utilizing multifilament mechanisms wherein a new filament can be inserted into the system either manually or automatically. Here again, the parameters of the individual filaments change due to the manufacturing tolerances.

However, there remains always the problem of supplying a suificient electron emission for proper operation of the beam device. Previous solutions used have involved supplying current at a sufi'iciently high level for emitting more electrons than needed, then in some manner reducing the electron flow in the beam to the desirable level. Naturally, this is an inefiicient operation of the filainent and filament life is shortened unduly by the higher emission and resulting greater evaporation rate.

The primary object of this invention is to control the filament current of an electronic device to obtain optimum filament life while always meeting the minimum requirements for electron emission from the filament.

Another object of this invention is to control automatically the filament current of an electronic device in response to filament emission while obtaining sufiicient electron flow and the longest life possible from the filament.

In accordance with the invention, an automatic filament current control is provided which varies or dithers the electric current supplied to the filament between predetermined maximum and minimum levels, while continually detecting the resulting change in electron flow, which change is compared to a predetermined reference signal for readjusting filament current to operate the filament always at a predetermined optimum point on the filament operating curve for the filament, thereby to obtain a combination of a optimum emission and a maximum useful operating life from the filament.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings:

FIGURE 1 illustrates an electron beam column in diagrammatic form with a schematic drawing of the filament current control;

FIGURE 2 shows a filament operating curve illustrating graphically the manner in which the electron beam current of the electron beam column of FIGURE 1 varies as the filament current is varied; and

FIGURE 3 shows graphically the manner in which certain operating voltages in the filament current control vary as the filament current is changed. I

In FIGURE 1 is illustrated diagrammatically and schematically an electron beam device and a filament current control utilizing the subject invention. In this drawing, an electron beam 10 is generated in an electron beam device 11 by supplying heating current to the terminals 12 of the cathode or filament 14. The electric current passed through the filament heats the filament to raise the thermal agitation of the material causing electrons to be emitted therefrom. Some of these electrons pass through an opening in a grid 15 maintained at a constant negative voltage potential relative to ground potential by means of the electrical connection made through the conductor 15a. The electrons thereafter are passed through an anode 16 maintained at ground potential to form the electron beam ,10 which is directed along the axis of the column 11 to strike a target or memory ele ment 17.

As the electron beam passes through the column 11, it is focused and reduced in size as well as being modulated to be used for recording data on the memory element 17. The beam is focused in the well known manner by a magnetic field of a first lens 18 including an annularly shaped electromagnetic coil 19. The beam then passes through the aperture of an aperture plate 20 and again is focused by the magnetic field of a lens 21 having an electromagnetic coil 22. After passage through the aperture of an aperture plate 24, the beam again is focused by a magnetic field formed by the lens 25 including the electromagnetic coils 26 and 27. The beam thereafter is scanned across the memory element by magnetic fields effected by the selective energization of the deflecting coil 28.

For modulating the beam such as is required for recording digital data on the memory element, the electrostatic plates 29 are energized in a predetermined sequence indicative of the data. When these plates are energized an electrostatic field is formed therebetween which deflects the electron beam sufficiently to prevent its passage through the aperture of the aperture plate 29a thereby preventing the beam from striking the memory element. Thus, by scanning the beam and selectively energizing the electrostatic plates to alternately prevent and permit to strike the memory element, digital data is recorded on the element. After the beam passes through the memory element 17, it strikes an electron sensitive PN junction device 30 which connects with ground.

The filament current I for heating the filament is supplied by the low voltage DC power supply 31 which is series connected through a proportional switch 32, including the transistors 33 and 34 and the current limiting resistors 35 and 36, across the terminals 12 of the filament. Thus, a direct current is passed through the filament having a magnitude responsive to the electrical conduction through the switch 32, which conduction is directly responsive to the potentials of the bases of the transistors 33 and 34 of the switch to transmit the direct current I and I which combine to form the filament current I FIGURE 2 shows a normal operating curve for a filament of the type used in such beam devices 11 indicating the manner in which the electron beam current 1 emitted from the filament varies in accordance mith the magnitude of the filament current. Note that as the filament current is increased the electron beam current increases at a substantial rate until the filament current reaches a magnitude indicated approximately by a point 37, at which time the rate of increase in beam current decreases until at point 38 there is very little increase of electron beam current or emission from the filament with a further increase in filament current. This curve is characteristic of most filaments and shows that after the beam current reaches a certain magnitude, the grid effect resulting from the feedback of a voltage signal to the grid and the space charge efiect resulting from driving the filament harder, both serve to severely limit the additional electron flow attainable by increasing the filament excitation. The existence and cause of these elfects is well known.

It also has been known that the most efiicient operation of the filament is obtained by setting the operating point thereof in the range just below .the knee of the characteristic curve, or between the points 37 and 38 on the curve of FIGURE 2. However, this cannot be accomplished merely by setting the filament current at a constant value since the curve shifts to one side or the other by variable amounts as each filament ages and the rate of emission changes. As pointed out before, the problem is further amplified in devices such as electron beam recorders where an automatic filament changing apparatus is included for injecting a new filament into the system when the old filament is no longer useful. The emission of each filament varies primarily because of the tolerances present in manufacturing. However, it should be noted that while the curve does change, from filament to filament, the change usually takes the form of shifting the curve laterally such that greater or less emission is obtainable for a given heating current, and the overall shape of the curve remains substantially the same.

In accordance with the present invention, a control is provided for setting the operating point of each particular filament to preselected optimum conditions automatically, which control superimposes a dithering signal on the normal filament heating current signal to vary the current supplied to the filament between predetermined maximum and minimum levels, senses the change in electron beam current emitted from the filament during this dithering, and compares the magnitude of the change in electron beam current to a reference signal to generate an error signal for adjusting the operating point of the filament heating current supply to operate the filament within a predetermined range on the characteristic curve and obtain optimum filament emission with the best possible filament operating life.

Accordingly, as shown in FIGURE 2, a dithering current I is superimposed on the normal filament current signal (represented as point 40) causing the operating point of the filament to vary to either side of the normal operating point 41 on the curve between the points 37 and 38. Wherein the normal electron beam current for a filament current indicated by point 40 would be that indicated by point 42 the electron beam current now is varied between points 44 and 45 in response to the dithering of the filament heating current. As can be seen by studying the curve of FIGURE 2, there is only one point near the knee of the curve which renders a preselected magnitude of variation in the electron beam current when a predetermined dithering signal 39 is superimposed onto the filament heating current signal since the slope of the curve at this point is always decreasing as the current magnitudes increase. This control detects the change in electron beam current occurring when the filament current is dithered and compares the change with a reference signal to generate an error signal for adjusting the heating current supplied to the filament for operating precisely at a preselected point on the filament operating curve for optimum emission and and operating life.

In FIGURE 1, is shown the schematic diagram of one embodiment of a control for achieving the heretofore described regulation of the filament heating current. A dithering signal is supplied by the oscillator 44 to the base of the transistor 33 of the switch 32 for superimposing on the filament heating current a dithering current I Thus, the dithered current I including the alternating current I passes through the transistor 33 and the current limiting resistor 35 while the variable operating current I for the filament is conducted through the transistor 34 and the current limiting resistor 36. Thereafter the currents are joined for transmission through the filament and subsequent return to the low voltage DC power supply 31.

As a result of the dithering signal, the beam current 1;; varies between a maximum and minimum level The circuit for detecting the beam current 1 connects with the filament circuit at the junction 46 and includes the conductor 47 connecting in series the resistor 48, the parallel-connected capacitor 49 and resistor 50, and the high voltage power supply 53. The return circuit to the power supply 53 for the beam current reaching the detector 30 is formed by the conductor 51 connected to the same ground circuit as is the detector of the electron beam column. The cathode 14 is maintained at a negative voltage potential relative to the anode potential by the circuit including the resistors 48 and 49 and the high voltage power supply 53 for accelerating the electrons in a direction away from the filament in the normal manner. In the present embodiment, the cathode is maintained at approximately 11,000 volts potential. Additionally, it must be recognized that all current emitted from the cathode does not reach the detector 30 in the normal operating manner of electron beam columns, since portions of the beam current are intersected by the aperture plates of the column, etc. Thus, as the dithering current I plus the preset heating current I is supplied to the filament, a constantly varying beam current I is emitted which is conducted through the resistor 48 to generate an alternating voltage V Byvamplifying, rectifying and filtering the signal V a voltage signal V is generated which subsequently is compared with a reference signal to determine if the filament is being operated at the preselected point on the operating curve. To measure V the voltage signals are detected at the junctions 55 and 56 to either side of the resistor 48 and, through the capacitors' 5,7 and 58, fed-to .a differential amplifier 59 which serves to detect the differential voltage across the resistor which is designated V The differential voltage signal V is amplified and fed to the full voltage rectifier and filter 60 which, in turn, supplies a direct current signal V -to a summing amplifier 61. Also fed to the summing amplifier 60 is a reference voltage V indicative of the desired change in the voltage V with the value of the reference voltage being determined by the point on the operating curve of FIGURE 2 it is desired to have the filament operate. For this purpose, a direct current supply or battery 66 is connected to energize a rheostat 67 for generating the reference voltage V which is fed to the summing'amplifier 61. Thus, the signal V indicative of V A received from the rectifier 60 and the signal V are compared in the amplifier 61 for generating an error signal V which is fed through the resistor 68' and an operating amplifier 69, having a feedback circuit including a parallel combination of a capacitor 70 and resistor 71, through the line 72 to the base of the transistor 3'4 thereby setting the magnitude of the current I passed through the transistor 34 to be added to that current I conducted by the transistor 33 to form the total filament operating current I Thus, a signal indicative of the voltage V always is added to a preselected reference voltage V for determining the magnitude of the filament current to obtain a predetermined change in the bearri. current 1 when the filament current is dithered the predetermined amount determined by the signal generated at the oscillator FIGURE 3 illustrates graphically the manner r in which the feedback signal voltage V is used to set thefilament current I The curve 74 indicates the manner in; which the error voltage signal V is changed to set the filament current magnitude. Actually the curve 74 is aldrivative curve indicating the slope of the curve of FIGURE 2 since the voltage V is proportional to the {magnitude of the change in beam current I as the fil-ament heating current is changed. I

The operating characteristics of the filament control feedback circuit are indicated by the line 75 withithe slope of this line being indicative of the gain of the'flfe'edbaek circuit. The operating points.of the filament control are determined in the normal manner by the points of intersection of the lines 74 and 75, with the intersection point 76 being preselected to set the filament current at a value for emission of the desired beam current 1 However, since the lines 74 and 75 also intersect at points 77 and 78 provisions must bernade to prevent the control from locking on one of these points. It is readily obvious that point 78 represents an unstable point since any increase in either the feedback signal V or the filament current I;- will cause a further increase in the magnitude of both signals.

The point 77 represents a stable operating point, however, the control ispreveuted from looking on this point by use of the switchihg element 79 (FIGURE 1). This switch 79 may be one of many well known designs which connect the terminals 80 and 81 when the voltage sensed at a third terminal 82 is below a preselected value, and disconnect these terminals when the terminal 82 voltage is above the preselected value. The voltage of the juncture 84 is fed to the terminal 82 as the preselected voltage below which the switch is closed. A source (not shown) for supplying a voltage V of opposite polarity to the Voltage V is connected to terminal 81. Thus, for values of I below, a specific magnitude, the switch 79 is closed to supply the voltage V;- to the summing amplifier 61.

FIGURE 3 illustrates how the supplying of the voltage V to the feedback circuit prevents the control from locking onto the stable operating point 77. As the voltage V is supplied to the summing amplifier 61, the operating characteristic line is shifted in the negative direction by the ;value V to the position of the dotted line 75a. Thus, the intersecting point 77 is eliminated. When I reaches a value (measured at juncture 84) to set I above the magnitude indicated approximately by the intersecting point 78, the switch 79 will close and the control will continue to function without the signal V being supplied. Therefore, the line 75 now represents the feedback characteristic of the control.

Thereafter, the summing amplifier receives the voltage signal V and V to set the filament current at the value indicated by point 76. Naturally, the point can be shifted (such as would be necessary to change the beam current level) to change the set point of filament current, by changing the magnitude of the reference voltage V To change V the setting of the rheostat 67 is varied. Changing the magnitude of the reference voltage serves to shift the line 75 vertically in a direction parallel to the V axis of the curve of FIGURE 3 to change the location of the operating point 76. Thus, the control operates always to sense the magnitude of the voltage V generates the feedback voltage signal V and adds to the signal V the reference voltage signal V to generate the voltage V for} setting the operating level of the transistor 34 of the switch 32. By this manner the filament operating point is set always to correspond with the predetermined operating point 76 of FIGURE 3.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it 'will be understood by those skilled in the art that various changes in form and details may be made therein withoiit departing from the spirit and scope of the invention.

We claim: 1. A control for regulating the electric current supplied to a filament for effecting the emission of a particle beam in a beam device, comprising:

an electric current source for supplying electric current :zft a variable magnitude to said filament,

means for superimposing on said filament current a constant frequency signal for dithering the magnitude of said filament current,

sensing means for measuring the change in particle emission of said filament resulting from the dithering of the filament current,

means for comparing the change in particle emission with a preselected signal related to a desired magnitude of change to generate an error signal indicative of the difference therebetween; and

means for adjusting the magnitudes of said current supplied to said filament to equalize the measured change with the preselected change in particle emissron.

2. A control as defined in claim 1 wherein the magnitude of electric current received by the filament from said current source comprises a preset magnitude of current which can be varied in magnitude by said adjusting means and on which is superimposed said dithering current which varies in magnitude between predetermined limits and at a preselected frequency.

3. A control as defined in claim 2 wherein said particle beam comprises a beam of electrons emitted by said filament.

4. A control as defined in claim 3 wherein said current source comprises an electric power supply providing electric current to said filament at a magnitude controlled by an analog switch actuated responsive to said error signal and to a constant frequency signal for dithering the magnitude of the current supplied to the filament.

5. A control as defined in claim 4 wherein said sensing means comprises a resistor through which the beam electrons are conducted as electric current and the change in filament emission is measured by sensing the change in the voltage across said resistor.

6. A control as defined in claim 5 wherein said comparing means comprises a summing device connected to receive a signal responsive to the change in voltage across said resistor and to receive said preselected signal related to a desired magnitude of change of electron emission with the output of said amplifier being used as the error signal to actuate said analog switch.

7. A control as defined in claim 6 including means for supplying to said summing amplifier a constant magnitude reference signal so long as said filament current is below a predetermined magnitude.

8. A control as defined in claim 1 wherein said error signal is set at a predetermined magnitude until the magnitude of filament current reaches a preselected value.

References Cited 15 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

